20Dating his brother's ex-girlfriend
When the show started out Peyton, Lucas’ constant love interest throughout the show, hadn’t been with him. Instead, Peyton’s boyfriend had been Lucas’ younger brother Nathan. At that time, there were a lot of lingering stares from both Peyton and Lucas toward each other and it was clear they were
into each other.
However, despite Peyton leaving Nathan and then getting together with Lucas, nothing can erase the fact that she used to be her brother-in-law’s ex-girlfriend. Not exactly something to tell the kids.